Transistor power amplifiers, particularly radio frequency power amplifiers, are subject to damage due to "punchthrough" and other related phenomena caused by high voltage levels at the device collector. The narrow base width of a high frequency transistor can be bridged by the collector voltage generated space charge region of the collector-base junction and effectively short the collector to the emitter allowing large currents to flow.
Modern devices, when operated within their limits, can withstand high collector supply voltages in combination with instantaneous peak voltages created by load mismatch when the power transistor is driven by the preceding stage at the rated drive power. When the supply voltage for the preceding lower power stages is high, the resulting high drive power in conjunction with the power transistor high supply voltage and the load mismatch voltage will often cause the power transistor collector to reach the punchthrough voltage and cause device damage. The present invention provides, for the first time, power semiconductor device protection in consideration of these damaging combinational effects.
One traditional method of protecting power amplifier transistors via a feedback control loop employs directional couplers for independent forward and reverse power detection. Detection of excessive forward or reverse power and reducing drive or power device supply voltage has provided good device protection at relatively high cost.
Another traditional but lower cost feedback method eliminates direct power detection and substitutes the sensing of power transistor current or voltage for an indication of the power dissipation of the device. This sensed parameter is used to control the output power of the transistor and is usually implemented by controlling the drive to the power device.
The lower cost method works well under matched load conditions, but a mismatched load will create standing waves which may give an erroneous indication of device operation. This erroneous indication may give rise to the feedback loop causing an increase or decrease in output power depending upon the standing wave current or voltage value at the sensing point. Thus, a device in a feedback loop employing a current or voltage sensing method to maintain a constant power output can be subject to even greater drive than a device not placed in such a loop.
Accordingly, a primary object of this invention is to protect a power amplifier semiconductor device, inter alia, from punchthrough damage.
Another object of this invention is to further avoid damage caused by high supply voltage, high reflected power due to load mismatch, and high drive level.
Another object of this invention is to correlate the protection of the semiconductor device to the supply voltage.
A more particular object of this invention is to foldback the semiconductor device input drive power by an amount greater than that power produced by the supply voltage in excess of a predetermined value.